**6. Conclusions**

This study covers the limitations of early-age strength development of concrete through an analysis that incorporates different test methodologies. The relationship of the maturity method and the ultrasonic wave propagation is that the hydration temperature results become higher in the initial hours of the hydration process; the wave propagation is slow as the concrete mix paste changes from a liquid paste to a semisolid form, the observed fluctuation of the waves was due to the medium form variation. The results predicted by the data, illustrated in Figure 13, are promising for concrete early-age strength monitoring and strength development estimation. The ultrasonic wave-propagation experiment provided a massive amount of data for predicting the compressive strength of undisturbed field-poured concrete as an NDT function, yielding a modified equation that gives a comparative result for strength estimation, which allows engineers a quick and safe project implementation process.

The data sugges<sup>t</sup> that the behavior of the signals and time of wave propagation to the boundary of the higher rigidity in the concrete sample results in a higher value of the wave propagation velocity.

For the maturity of concrete, the datum temperature plays a basic role in the results of the maturity index, as the maturity method result gave a smooth output and was a little higher than that of the ultrasonic waves. So, for each mix, the datum temperature should be defined according to mix proportions, as the hydration temperature directly affects the strength development. Furthermore, from the results, it has been concluded that high temperature during concrete curing causes rapid strength gains in the structural elements.

The results qualify the proposed method for structural health monitoring in a wide range of industrial multi-stepped project monitoring. The procedure can be advanced in the future for the integration of methodology to the equipped systems for synthesis in the modified polymers.

**Author Contributions:** N.T. methodology, data collection and analysis, and manuscript writing; J.K. support, data collection, and experimental support; W.-K.K. methodology, data collection, and experimental support; S.P. guidance and support.

**Funding:** This research was funded by the Korea Agency for Infrastructure Technology Advancement (KAIA) gran<sup>t</sup> funded by the Ministry of Land, Infrastructure and Transport (Grant 19CTAP-C151808-01), the National Research Foundation of Korea (NRF) funded by the Korean governmen<sup>t</sup> (MSIP; NRF-2017R1A2B3007607, NRF-2017-R1D1A1B03033399), and it was financially supported by the Korea Ministry of Land, Infrastructure and Transport (MOLIT) as theǍInnovative Talent Education Program for Smart Cityǎ.

**Acknowledgments:** This work was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) gran<sup>t</sup> funded by the Ministry of Land, Infrastructure, and Transport (Grant 19CTAP-C151808-01), by grants of the National Research Foundation of Korea (NRF) funded by the Korean governmen<sup>t</sup> (MSIP; NRF-2017R1A2B3007607, NRF-2017-R1D1A1B03033399) and it was financially supported by the Korea Ministry of Land,InfrastructureandTransport(MOLIT)astheǍInnovativeTalentEducationProgramforSmartCityǎ.

**Conflicts of Interest:** The authors declared no conflict of interest.
