# Determination of Concrete Formwork Removal Time Based on Ultrasound Reflection
## 4 Results  
Figure 6 shows indicative waveforms at different times after casting, specifically, 3 h, 10 h and 14 h. The recorded waveform certainly includes the reflection from the interphase, but simultaneously carries other contributions, such as the surface wave travelling directly from the pulser to the receiver on the external side of the buffer, other reflections from the boundaries of the cylindrical buffer and, certainly, the resonant behavior of the sensor. The combination of the resonant behavior and the relatively low frequency does not allow accurate separation of the different contributions within the waveform and this is the reason that numerical simulations are also discussed later on. The major contribution comes after 50 \(\upmu\)s being in agreement with the expected wave reflection coming from the interface considering the buffer thickness of 50 mm and the buffer UPV of 2500 m/s. Another strong burst is exhibited between 100 and 150 \(\upmu\)s, with a lower amplitude, presumably from the 2 nd reflection. The amplitude of these bursts is not constant with time. At 9 h (Figure 6b) both bursts seem to obtain a lower value than at 3 h (Figure 6a) and at 12 h (Figure 6c). Specifically, the 2 nd burst after 100 \(\upmu\)s stands at approximately 1.55 V (peak to peak) at 3 h, see Figure 6a. At 9 h it is lower at about 1.32 V (Figure 6b). Later at 12 h, it is again higher reaching approximately 1.69 V (Figure 6c). For later times it reaches an amplitude higher than 2 V. Therefore, the hardening of the concrete itself without any other change in the setup plays a very significant role and influences the amplitude of the received signal. Other wave components, such as the direct Rayleigh, or reflections from the cylindrical boundary, are not expected to change as they are not influenced by the hardening of concrete; this is why the contribution until almost 50 \(\upmu\)s is identical to all waveforms. Therefore, the fluctuation of the amplitude with time can be safely attributed to the impedance changes in the hardening medium. It is again noted that even when the impedances of the buffer and concrete are equal, still the receiving transducer will record a certain level of amplitude due to the different aforementioned contributions; therefore, the point of interest is when the amplitude presents a minimum and not absolutely zero. Figure 5: Pulser-receiver pair on a preliminary experiment. Figure 7a shows the receiver's peak amplitude (2 nd burst between 100 and 150 us) history for the case of the short model specimen (a) and the 3 m real-sized formwork (b). The curve of (a) shows that at 5 h after casting, the amplitude of the reflected wave started to decrease until it reached a minimum at approximately 10 h. This point indicates that the impedance of the buffer material was very close to the impedance of the concrete at that time. After 10 h, the concrete continued to gain stiffness and its impedance exceeded that of the buffer material resulting in a higher reflection again. Moving to the real-scale formwork, the corresponding results are depicted in Figure 7b. A similar trend is seen when at approximately 9-10 h after casting, the receiver's amplitude undergoes a local minimum. This validates the initial result that this is the time when concrete obtains the impedance of 4.15 MRayl similar to the epoxy of the buffer, and the Young's modulus of 5 GPa. Other smaller fluctuations in the amplitude curve are also observed, without a clear explanation, such as the initial increase of the received amplitude between 0 and 4 h in Figure 7a. These transient fluctuations may be due to changes in the local composition of the material behind the point of measurement (settlement of the material, air, water and aggregate mobility). However, both monitoring cases point to the time between 9-10 h as the period of minimum reflection and thus equal impedance between buffer and hardening concrete.