**4. Results and Discussions**

The internal strains of the specimens in X, Y, and Z directions were recorded from the casting date. The results are presented up to 200 days in Figure 4. The data is presented from 51 days in the figures since external sensors were attached in this day and all the figures in this paper remain in a same time scale for a convenient comparison. Figure 4a shows the average expansion along the specimen dimensions for the reactive specimens and the average volumetric expansions for both reactive and control specimens are presented in Figure 4b. The results for volumetric strains show that reactive specimens were expanded. Conversely, the control specimens had no expansion and the internal strain was relatively constant, only showing small shrinkage over time. However, the volumetric strains for both confined and unconfined specimens were very close. This observation indicates that an imposed confinement in the specimens did not influence the expansion of the entire specimens (volumetric strain). Furthermore, the confinement changed the direction of expansion (Figure 4a).

**Figure 4.** Strain distribution of specimens versus concrete age. (**a**) Directional expansion; (**b**) Volumetric strain.

Generally, the expansion through the thickness of the reactive specimens (Z direction) is much larger than the expansion throughout the X-Y plane, due to partial or complete confinement in the plane and lack of steel reinforcement through the thickness. The maximum strains in the Z direction for confined and unconfined specimens are approximately 3.6 and 2.1 times the strain in the Y direction, respectively, at 200 days. The steel frame in the confined specimen causes a reduction of both in-plane strains. In the confined specimen, the strain along the X direction is less than the Y direction, while in the unconfined specimen, the strain along X and Y are almost the same. The strain along the X direction in the confined specimen is 58% of the strain along the X direction in the unconfined specimen at 200 days. However, the strain along the Y direction in the confined specimen is 84% of the strain along the Y direction in the unconfined specimen at 200 days. Moreover, the confinement in X-Y plane for the confined specimen caused an increased strain rate along the Z direction (thickness). The expansion strain rate in terms of time decreases at the point around 150 days, which is named inflection point as seen in Figure 4. The inflection point in volumetric strain is considered as one of the important point for ASR modeling, where the curvature of volumetric strain is changed [27,28]. The latency and characteristic times (two modeling parameters) are experimentally determined by knowing the location of inflection point [6,28]. The inflection point is shown in Figure 4b and marked in other figures in this paper.

Acoustic emission data was recorded through the internal sensors from the casting day and resonant sensors started recording from the concrete age of 51 days. Filtering AE data is an important step for reducing the amount of non-relevant data for post-processing. The possible sources of false AE data can be friction between the structural components, such as the steel frame and specimens, and water dripping from the chamber ceiling due to high humidity. Two different filtering procedures were developed to minimize the non-genuine data for the internal and external sensors, separately. The filtering is different for the internal and external sensors due to their differences in sensitivity and location. For instance, the internal sensors are much less sensitive than the external sensors and were located inside the specimens, thereby receiving less environmental noise. The AE data below 32 dB and 41 dB for the internal and external sensors was filtered from the data set. Then, the signals were further filtered using a Swansong filtering procedure [29,30]. This method is based on the observation that genuine AE signals with high amplitude should have long duration and vice versa [11]. Therefore, false signals in this method are categorized by long duration with low amplitude and short duration with high amplitude. The data is presented in terms of duration versus amplitude distribution. Signals which did not comply with the characteristics of genuine signals were deleted based on visual observation of the waveforms. The rejection limits for the internal and external sensors are presented in Table 1. In addition, suspicious signals were removed by inspection of waveforms and the chamber activity timetable provided by the University of Tennessee, Knoxville. The filtered AE data from 50 to 195 days after casting for all sensors is presented in Figure 5. The figure illustrates the amplitude and cumulative signal strength (CSS) in terms of specimen age. The time window of 50 to 195 days was selected because the highest strain change rate occurred during this period. The gap in the data from 65 to 100 days is missing data due to a difficulty with the data acquisition system caused by an unexpected energy surge. The amount of data in the control specimen is much lower than the data for the reactive specimens. The difference between AE data for the control and reactive specimens is also observable from plots of cumulative signal strength versus time (Figure 5). Therefore, the relatively high AE activity for the reactive specimens can likely be associated with expansion caused by the alkali-silica reaction.

**Figure 5.** Amplitude and cumulative signal strength of AE data in terms of concrete age. (**a**) Confined specimen; (**b**) Unconfined specimen; (**c**) Control specimen.


**Table 1.** Duration-amplitude rejection limits.

#### *4.1. Acoustic Emission Energy Release*

To enable comparisons between the confined and unconfined specimens only AE data recorded by the internal sensors is discussed. These sensors are less sensitive than the resonant sensors, which results in lower volumes of data. The main reason for choosing the broadband sensors is that their broad frequency response makes them suitable for frequency analysis. The resonant sensors attached on the bottom surface of the specimens are more representative of what may be used in practice for optimized detection and source location. Three dimensional source location of large-scale specimens requires a method to accurately calculate time of arrival for very weak signals, which is a future step of this research. This study involves frequency analysis and therefore focuses on broad band sensors. Analysis of the resonant sensor data is a future consideration.

In Figure 1, the red marks and corresponding labels in red font denotes the schematic sensor locations for both confined and unconfined specimens. Sensor coordinates are presented in Table 2. The last column of the table represents the coordinates of normal vectors, which are perpendicular to the sensing surfaces of sensors and the directions of vectors are toward the outside of the sensors. The orientation of sensors is shown in Figure 1d.


**Table 2.** Sensor coordinates.

The AE cumulative signal strengths of the internal sensors for the reactive specimens are presented in Figure 6a,b. The total CSS versus time for the confined specimen is much higher than the CSS for the unconfined specimen at 195 days (the CSS for the confined specimen is 2.35 times the value for the unconfined specimen). Moreover, in Figure 5a,c, it can be seen that a significant portion of released AE energy is attributed to sensor 3, which is located at mid-height of the confined specimen. The CSS rate for sensor 3 (5805 pVs/Day) is much larger than for sensors 1 and 2 (1545 and 1212 pVs/Day), thereby increasing the difference in the CSS between the sensors. On the other hand, this trend in the confined specimen is not observed in the unconfined specimen where AE energy release is not concentrated in one specific sensor. The distribution of energy was approximately uniform among the sensors. However, the CSS for the sensors at the top reinforcement mesh (sensors 4 and 5) is larger than in sensor 6.

**Figure 6.** Cumulative signal strength (CSS) contribution for sensors. (**a**) CSS vs. age of concrete for confined specimen; (**b**) CSS vs. age of concrete for unconfined specimen; (**c**) Normalized CSS for confined specimen; (**d**) Normalized CSS for unconfined specimen.

These observations illustrate that the confined specimen has larger AE energy release in the middle layers of the specimen, which is increasing with the progression of the ASR reaction, than the unconfined specimen. This may be due to a larger expansion strain through the thickness of the confined specimen. This large expansion is expected to cause more damage through the thickness of specimen and consequently more AE energy release at middle of the thickness. In the unconfined specimen the crack distribution is expected to be less anisotropic than in the confined specimen. Therefore, it is expected that the AE energy was more uniformly distributed through the thickness of this specimen.
