Series on the topics of fly ash concrete The 5th topic: Assessment of Alkali Aggregate Reaction  In 2007, the Concrete Committee of JSCE established a subcommittee on design and application of fly ash concrete to further promote the effective use of fly ash in accordance with the performance-based design. The subcommittee will report the activity of six working groups at regular intervals in a newsletter in 2009.     Fly ash helps prevent alkali aggregate reaction. This advantage can be efficiently applied through the use of a guideline. The committee investigated the influence of the quality of the fly ash, the reactivity of the aggregate, and the condition of the alkali on the fly ash’s ability to prevent alkali aggregate reaction. Mortar bar tests showed that the JIS classification could be used to assess this ability. This conclusion is supported by the strong relationship between the expansive strain and the total surface area of amorphous silica. (Figure 1) The committee also conducted concrete bar tests using several reactive aggregates, which further confirmed the preventive effect of fly ash.     When concrete structures are electrochemically desalinated to prevent reinforcement corrosion, the concrete’s alkali content may become high, increasing the risk of an alkali aggregate reaction. Fly ash reduces the degradation even when the alkali content is extremely high, indicating that the preventive mechanism of fly ash works not only by reducing the alkaline content but also by replacing the cement with fly ash. As a result of the tests described above, the committee has proposed a replacement ratio for fly ash. Alkali aggregate reaction is caused by the reactivity of the aggregate, the reactive mineral content, and the alkaline content in the concrete. In some countries, several preventive measures are applied depending on the risk level, which is determined by the reactivity of the aggregate and the environmental conditions. However, this grading system is difficult to apply in Japan because the reactivity of the aggregate is unpredictable due to the complexity of Japan’s volcanic geological structure.     First, the committee classified the levels of countermeasures as “normal” and “high.” Table 1 shows several theoretical cases and the level of countermeasures to be applied. The “normal” level is for preventive maintenance.     Second, based on tests and research, the committee proposed the use of fly ash. Table 2 shows the recommended replacement ratio for fly ash. The replacement ratio depends on the quality of the fly ash and the level of countermeasures. The former is classified in accordance with the Japan Industrial Standard (JIS A 6201). Note that the JIS does not consider the CaO content. The fly ash currently produced in Japan rarely contains more than 8% CaO. Therefore, the recommended replacement ratio shown in Table 2 should be modified when fly ash with a high CaO content is produced in the future.     Because the alkali aggregate reaction strongly depends on the alkaline content of the concrete, the preventive measures also depend on the alkaline content. When the risk is higher due to the presence of foreign alkaline ions, the replacement ratio for fly ash should be higher than the value shown in Table 2. Figure 1 Relationship between expansive strain and surface area of amorphous silica in fly ash Table 1. Several theoretical cases and the level of countermeasure to be applied  Theoretical Case  Level of Countermeasure  Highly reactive aggregate and highly reactive mineral content  High level  To ensure protection from ASR (e.g.,important structure)  High level  Difficult to predict aggregate reactivity and amount of and variation in reactive minerals  High level  Likely to contain little reactive aggregate  Normal level  Total alkaline content in concrete is higher than the regulatory value (3 kg/m3)  Normal level   Table 2. Recommended replacement ratio for fly ash to prevent ASR  Quality of fly ash  Normal Level High Level   JIS type II or more, JIS type IV (surface area: 2,500 cm2/g or more)  15 % or more 20 % or more   Others  30 % -