## 3 Unconfined Compressive Strength Test
### Analysis of Unconfined Compressive Strength Test Results  
The specimens were placed in a standard curing room continuously for 3 days, 7 days, 14 days, 28 days, 60 days and 90 days. The unconfined compressive strength test was carried out. The test results of cement-treated aggregate base material with different curing times are shown in Table 3.  
From the test results for unconfined compressive strength presented in Table 3, it can be noted that the unconfined compressive strength of the cement-treated aggregate base material varied dramatically in both curing times and loading rates. The pattern of variation was described by a logarithmic equation, and the strength test resulted in different curing times and different loading rates, fitted by Equations (2) and (3), respectively. The fitting results are shown in Figure 5, where \(R^{2}\) is the correlation coefficient of the fitting.  
\[S_{c}=a_{1}+b_{1}In(t+c_{1}) \tag{2}\]  
\[S_{c}=a_{2}+b_{2}In(t+c_{2}) \tag{3}\]  
where \(S_{c}\) is the compressive strength; \(t\) is the curing time; \(v\) is the loading rate; \(a_{1}\), \(a_{2}\), \(b_{1}\), \(b_{2}\), \(c_{1}\), and \(c_{2}\) are the fitting parameters.  
Figure 4: The procedure of the unconfined compressive strength test (**a**) Specimens immersed in water (**b**) Selection of standard specimens (**c**) Strength test (**d**) Failure sample of unconfined compressive strength.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Curing Time (\(t\)/d)**} & \multirow{2}{*}{**Loading Rates**} & \multicolumn{6}{c}{**Unconfined Compressive Strength of Parallel**} & \multirow{2}{*}{**Average Value**} \\  & & & & \multicolumn{3}{c}{**Specimens S\({}_{c}\) (MPa)**} & & \multicolumn{3}{c}{**\(\overline{S_{c}}\) (MPa)**} \\ \cline{3-10}  & & **1** & **2** & **3** & **4** & **5** & **6** & **7** & \\ \hline
3 & & 3.1 & 2.7 & 2.8 & 3.2 & 2.6 & 3.0 & 2.8 & 2.9 & 7.58 \\
7 & & 3.4 & 4.3 & 3.5 & 4.0 & 3.6 & 3.9 & 3.4 & 3.7 & 9.36 \\
14 & 1 & 4.7 & 3.9 & 4.1 & 4.2 & 4.8 & 4.6 & 4.3 & 4.4 & 7.65 \\
28 & 1 & 5.1 & 4.3 & 4.3 & 4.7 & 4.9 & 4.4 & 4.6 & 4.6 & 6.70 \\
60 & & 5.5 & 5.9 & 5.7 & 5.4 & 5.2 & 5.3 & 5.5 & 5.5 & 4.33 \\
90 & & 5.9 & 5.8 & 5.5 & 5.6 & 5.3 & 6.0 & 6.2 & 5.8 & 5.40 \\  & 0.1 & 4.3 & 4 & 4.2 & 4.3 & 4.5 & 4 & 4.2 & 4.2 & 6.92 \\  & 0.5 & 4.3 & 4.2 & 4.7 & 4.6 & 4.6 & 4.4 & 4.3 & 4.4 & 7.57 \\  & 1 & 4.6 & 4.3 & 4.3 & 4.7 & 4.9 & 4.4 & 4.6 & 4.5 & 6.70 \\
28 & 3 & 5.2 & 4.8 & 4.7 & 5.1 & 4.9 & 5.0 & 5.1 & 5.0 & 3.60 \\  & 5 & 5.4 & 5.3 & 4.9 & 5.4 & 5.2 & 5.1 & 4.9 & 5.2 & 4.16 \\  & 10 & 5.3 & 5.2 & 5.0 & 5.6 & 5.1 & 5.4 & 5.2 & 5.3 & 7.46 \\ \hline \end{tabular}
\end{table}
Table 3: Test results of unconfined compressive strength with different curing times.  
From the fitting results, it can be noted that:  
1. The effect of the curing time on the strength of the cement-treated aggregate base material is larger than the loading rate. The strength value increases with an increased curing time. For example, the strength at a curing time of 90 days was two times larger than that of three days. The strength increased quickly with the curing time in the early period, and tended to be stable in the later period. In the first 28 days, the strength value increased by about 58.6% (from 2.9 MPa to 4.6 MPa). This is consistent with the findings in the literature [24, 25]. The strength increased slowly in the later period, therefore, the semi-rigid base layer should be maintained for more than 28 days before the upper layers are paved.
2. The strength of the cement-treated aggregate base material is affected by the test loading rate significantly, but much less than the curing time. The strength value increased by about 26% (from 4.2 MPa to 5.3 MPa) with the increased loading rate (from 0.1 mm/min to 10 mm/min). Therefore, when the compressive strength was taken as the pavement design parameter, the effect of the loading rate should be considered.