## 4 Results and Discussions
### Pattern Recognition of AE Data
This trend in signal frequency from low to high in the confined specimen is not pronounced before 150 days. The SSCF for cluster Cluster-3 is slightly greater than the Cluster-2 at 150 days. The signal frequency trend initiates primarily after 150 days in this specimen. On the other hand, in the unconfined specimen, the frequency content evolution of AE signals is obvious from an earlier stage of ASR reaction (66 days) and is more significant after 150 days. The confined specimen has a higher extensional strain along the Z direction than the unconfined specimen (approximately 42% more at 195 days). This expansion leads to tension concentration through the thickness of the confined specimen. Since there is no confinement through the specimen thickness it is susceptible to crack formation. In the unconfined specimen, the expansion strain is more evenly distributed between the X-Y plane and the thickness. Therefore, the tension is more uniformly distributed in the entire specimen in comparison to the confined specimen. This is also observable from the AE data, where sensor 3, located at the mid-height of the confined specimen, has a larger SSCF than the unconfined specimen (e.g., 65% for the confined specimen versus 35% for the unconfined specimen at 195 days). As mentioned previously, the frequency of AE signals progresses from low to high as the concrete ages. This may be attributable to the formation of cracking through the coarse aggregate due to ASR progression. The crack formation inside the aggregate is expected to have higher frequency components than the cement matrix and interfacial transition zone (ITZ) as mentioned by [PERSON] et al. (2017). The transition from low-frequency signals to the high-frequency signals in the confined specimen initiated later than for the unconfined specimen (after 150 days). However, there are different contradictory hypotheses relating to formation of cracks in concrete due to ASR ([PERSON] _et al._, 2016). For instance, osmotic pressure theory was proposed by [PERSON] to describe the mechanism of expansion ([PERSON] _et al._, 2016). In this theory, the cement paste surrounding to reactive aggregates acts as a semi-permeable membrane, which water solution can pass inside the region around the reactive aggregates, but alkali-silica ions are enclosed in the reactive regions. This causes osmotic pressure and alkali-silica gel swells and exerts pressure to the cement paste. This pressure leads to crack formation in the cement paste ([PERSON] _et al._, 2016). [PERSON] and [PERSON] also proposed a similar theory as osmotic theory, which transforming a solid alkali-silica layer on a reactive aggregate to a gel by absorbing moisture from the pore solution was explained as a main reason of cracking in the cement paste due to ASR ([PERSON] and [PERSON], 2016). [PERSON] and [PERSON] suggested that the cracking is caused in the cement paste and interfacial transition zone due to accumulation of alkali-silica gel in the interfacial transition zone and resulting gel pressure ([PERSON] and [PERSON], 2016). On the other hand, [PERSON] and [PERSON] assumed that crack formation occurred far away from reactive aggregates due to diffusing dissolved silica away from aggregate into the pores in the cement paste ([PERSON] and [PERSON], 2016). Some researchers observed that ASR gel initially forms inside a reactive aggregate and causes the pressure and crack formation inside the aggregate and surrounding cement paste ([PERSON] and [PERSON], 2016; [PERSON] and [PERSON], 2016; [PERSON] and [PERSON], 2016; [PERSON] _et al._, 2016; [PERSON] and [PERSON], 2016). [PERSON] and [PERSON] ([PERSON], 2016) related the cracking pattern of concrete due to ASR to the types of reactive aggregate. ASR cracks start to form inside aggregate or in the cement matrix depending on the aggregate type ([PERSON], 2016). In the confined specimen, the CSS rate of Cluster-3 (energy concentration 150-300 kHz), started to increase at the age of 150 days.