## 5 Conclusions  
This study analyzed the change of the air void structure and concrete property during the early stage. The ultrasonic scattering method was applied to measure the air void size distribution and to analyze the properties of both early-age and hardened concrete. The results were favorably compared with the ASTM C457 test results for validation. Based on the measured results, the change of the air void structure during the early age, and its relationship to concrete transport ability and the compressive strength were also analyzed.  
During the early stages, the decreasing trend of the cement paste attenuation can be observed with the curing ages. The air void volume fraction obtained from the ultrasonic scattering measurement was found to decrease in both the small and large air void range with the curing ages. These phenomena are caused by the change in cement paste volume during the continuous hydration process. The results obtained from the ultrasonic scattering measurement were further verified with the ASTM C 457 test results on Day 28. The correlation coefficient between the ultrasonic scattering back-calculated values and the ASTM C457 measured results on the whole size range are 88.86%, 96.88%, and 90.75%, for Sample Types 1-3, respectively. The relatively high correlation coefficients further indicate the feasibility of the ultrasonic scattering measurement methods.  
The electrical resistivity of concrete samples was found to increase with the curing age, which is in accordance with the air void shrinkage during the hydration process. In addition, the correlation between ultrasonic parameters and compressive strength was evaluated. The dynamic modulus was analyzed with the ultrasonic transmission speed, and the results are in good agreement (within 5% error) with the resonance-measured results. The relationship between the air void content and strength loss was also analyzed. The clustered air voids along the ITZ area can cause more severe compressive strength loss.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} **Mass Density at** \\ **Hardened Stage (kg/m\({}^{3}\))** \\ \end{tabular} & \begin{tabular}{c} **Ultrasonic Transmission** \\ **Speed (m/s)** \\ \end{tabular} & \begin{tabular}{c} **Predicted Dynamic** \\ **Modulus (GPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Measured Dynamic** \\ **Modulus (GPa)** \\ \end{tabular} &
\begin{tabular}{c} **Relative** \\ **Errors (\%)** \\ \end{tabular} \\ \hline Type 1 & 2.30 & 4612.9 & 41.5 & 43.3 & 4.2\% \\ Type 2 & 2.23 & 4488.4 & 39.2 & 37.5 & 1.6\% \\ Type 3 & 2.16 & 4024.2 & 31.6 & 31.7 & 0.3\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Properties of concrete samples during the early stages.  
This paper provides an ultrasonic scattering method to measure the air void size distribution and volume fraction during both early-age and hardened stages. The correlation between concrete properties and ultrasonic parameters was also analyzed. Thus, the ultrasonic techniques can be further developed to measure concrete air void evolution and property development during early stages for concrete construction inspection.  
The authors would like to acknowledge the Michigan Tech Transportation Institute Initiative Fund program for partial financial support. The first author acknowledges the financial support from China Scholarship Council under No. 201406370141. The authors would like to thank [PERSON] at Michigan Technological University for assistance in the ASTM C457 air void measurement.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the experiments and analyzed the data; [PERSON], [PERSON] and [PERSON] contributed the analysis tools; [PERSON] and [PERSON] wrote the paper.  
The authors declare no conflict of interest.