# Strength Correlation and Prediction of Engineered Cementitious Composites with Microwave Properties
## 5 Conclusions  
This paper presents the compressive strength prediction and correlations of engineered cementitious composites (ECCs) by means of early-age microwave conductance using a nondestructive microwave detecting technique. The measurements of effective conductance and compressive strength were undertaken in parallel for the same type of ECC specimens with different water-to-binder ratios. Near-field measurement was conducted daily in the first eight days whereas the compression test was performed in accordance with the standard ASTM-C39 till 28 days of age. Innovative models for the correlations of compressive strength against effective conductance were developed and analyzed. The following conclusions can be drawn within the scope of this study:  
1. The decay of conductance at 2 GHz is modelled by using the bi-exponential regression function (4), where the time-constant of the rapid decay term increases as a function of increasing \(w/b\).
2. The growth of compressive strength of ECC specimens till 28 days is found to be best fitted by using bi-exponential function (5). The time constant of the rapid growing term decreases as a function of increasing \(w/b\).
3. ECC specimens have the general trend--strength develops as a function of decreasing effective conductance. This produced the ability of the microwave technique in monitoring the variation of the liquid phase morphology and pore structure.
4. When considering the ECC specimens with similar \(w/b\) ratios in the range of 0.20-0.30, a generic model is developed in a bi-exponential form for all ECC specimens as a whole, as given in (6).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline ECC & \(w/b\) & \(K\) & \(m\) & \(R^{2}\) \\ \hline PE1 (\(f_{c1}\)) & 0.200 & 2.274 & 1.196 & 0.9814 \\ PE2 (\(f_{c2}\)) & 0.255 & 2.710 & 1.285 & 0.9953 \\ PE3 (\(f_{c3}\)) & 0.300 & 3.008 & 1.336 & 0.9979 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Regression coefficients and qualities for Equation (7).  
Figure 13: (a) S-C correlation for PE1 (\(w/b\) = 0.20) specimen; (b) S-C correlations for PE2 (\(w/b\) = 0.255) and PE3 (\(w/b\) = 0.30) specimens.  
An accurate correlation model for strength-conductance is developed by including \(w\)/\(b\) as the dependent variable. The rational correlation model effectively emulates the nonlinear evolution of compressive strength with decreasing conductance. Moreover, the two coefficients \(K\) and \(m\) increase as a linear function of increasing \(w/b\). This makes in situ nondestructive strength predicting and monitoring feasible for concrete infrastructure.  
The authors would like to express sincere gratitude to [PERSON] for his kind assistance on the project and valuable comments on the revised manuscript. The research is supported by the National Natural Science Foundation of China (Project No. 51678322 and 51650110509), and the Taishan Scholar Priority Discipline Talent Group program funded by the Shan Dong Province.  
[PERSON] and [PERSON] conceived and designed the ECC experimental works; [PERSON] performed the experiments whereas [PERSON] and [PERSON] analyzed the data; [PERSON] and [PERSON] contributed manpower/materials/analysis facilities.  
The authors declare no conflict of interest.