*2.2. Measurement Equipment*

To measure the strain inside the specimens, two kinds of strain sensors, alkali-tolerant strain transducers (KM-100B from Tokyo Sokki Kenkyujo) and long-gauge fiber optic extensometers (from SMARTECH/Roctest), were employed. In total, sixty-four 100 mm-gauge strain transducers were placed inside the specimens, along with the specimen dimensions. The sensors were fixed on the support fabricated by smooth steel bars with a diameter of 3 mm. Additionally, five high-precision, fiber optic extensometers were installed inside the specimens. The sensors in the X-Y plane had 1.5 m and 1.8 m gauge lengths and were located at the bottom of the specimens. Two fiber optic sensors were installed along the specimen thickness with a gauge length of 0.8 m [22].

Seven acoustic emission sensors were utilized in each of the confined and unconfined specimens (reactive specimens) as shown in Figures 1 and 2. Two sensors were employed for the control specimen. The internal AE sensors were broadband WDIUC-AST (manufactured by MISTRAS Group, Inc., Princeton Junction, NJ, USA), with an operating frequency range of 200–900 kHz and an internal low-noise 40 dB preamplifier. The external sensors were resonant R6I-UC (manufactured by MISTRAS Group, Inc., Princeton Junction, NJ, USA), with an operating frequency range of 35–100 kHz and an internal 40 dB preamplifier. The special polymer coating on the sensors (internal and external) with an internal waterproof cable makes the sensors insulated, non-conductive, and capable of operating under water.

Three broadband sensors were installed inside the reinforcement cages prior to casting for reactive specimens and one sensor was installed in the control specimen. Sensors 1, 2, 4, and 5 were attached under the second layer of top reinforcement mesh (under reinforcement in the Y direction) with epoxy and fastened to the rebar by several cable ties and duct tape, with the sensing surface facing downward along the negative Z axis (Figures 1d and 2d). Sensors 3 and 6 were in the middle of the specimen thickness, with the sensing surface facing toward the negative X axis (Figure 1d). Sensors 3 and 6 were attached on the supports by epoxy and duct tape and cables were fastened on the support by cable ties (Figure 2c). The supports were fastened to the top and bottom reinforcement meshes. The locations of internal sensors were colored on the top reinforcements to avoid over-vibrating during casting and potentially decoupling the sensors.

Four resonant sensors were attached at the bottom of the reactive specimens and one to the control specimen (Figure 2b). The resonant sensors were attached with epoxy on the surface of the concrete and fixed by holders as shown in Figure 2b. Internal and external sensors are referred to as broadband and resonant sensors, respectively, in this paper. The holders were made of stainless steel and polyurethane pads were attached between the concrete and the holders to avoid potential corrosion in the holders.

External and internal sensors pose differing technical challenges. Internal sensors are of scientific interest and are not well-suited for implementation in existing structures. One potential technical consideration is the formation of voids around the sensors due to concrete shrinkage. This may result in interruption of data or reduction of sensitivity. Another technical challenge is lack of access after casting. The external sensors are accessible, but decoupling may be a consideration.

A 16-channel Sensor Highway II (SHII), manufactured by MISTRAS Group, Inc. (Princeton Junction, NJ, USA), was utilized as a data acquisition system. The sensitivity of external sensors was checked by applying Hsu-Nielsen sources [23].

The sampling rate was set to 1000 kHz. Pre-trigger time is the period required for data acquisition to save a signal prior to threshold intersection, which was set to 256 μs. HDT (hit definition time) is the time for which if a signal crosses the threshold the signal will continue until the end of HDT without stopping. HDT was set to 400 μs. HLT (hit lockout time) is the time defined at the end of a signal for neglecting any reflected signal that exceeds the threshold, which was set to 200 μs. PDT (peak definition time) is the time which ensures correct identification of the signal peak for rise time and peak amplitude measurements [24]. In this study, PDT was set to 200 μs. The initial threshold was 32 dB.
