*3.1. Shear Wave Velocity Testing*

It has been shown that the propagation of the shear wave is less affected by the presence of air void in cementitious materials. Thus, only the shear wave velocity is obtained and analyzed in this study. It was noticed that it is hard to pick the first arrival of shear waves from a single piece of signal in the time domain. Thus, B scan images were formed by a stack of signals in the time domain together and used to capture the shear wave arrival. The shear wave arrival time was first obtained using a MATLAB (R2019b, The MathWorks, Inc., Natick, MA, USA) input function to manually pick arrival points at different ages along the arrival time trend. Then, the shear wave velocity was calculated by dividing the thickness of the mortar sample, which was 27 mm in this study, over the travel time of the wave through the sample. Note that a test system time of 4 µs was subtracted to obtain the actual travel time.

Figure 2 shows a B scan image of wave propagation evolvement during setting in a mortar sample (w/c = 0.40), the x-axis and y-axis in the figure representing the time of signals and the age of mortar, respectively. It was noticed that at the first 2.5 h of setting, direct transmission waves transmit along the rubber mold are stronger than the waves propagating directly through the sample. However, by using B scan images, the first arrivals at a very early age can still be obtained.

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**Figure 2.** B scan image formed from signals getting from a mortar sample (w/c = 0.40).

Shear wave velocities obtained from different w/c mortar samples are shown in Figure 3, and the times of the initial setting are also marked on the corresponding curves. It is clearly seen that as w/c increases, a much longer initial setting time is taken, and it was also noticed that even though the w/c are different, the shear wave velocities are very consistent (around 410 m/s) at initial setting times. This trend agrees very well with previous research on cement paste [22].

**Figure 3.** Shear wave velocities in different w/c mortar samples.
