# Fracture Properties of Concrete in Dry Environments with Different Curing Temperatures
## 3 Results  
### Load-CMOD Curves  
Figure 3 presents the complete load-CMOD curves for concrete in a dry environment with different temperatures, which was used to obtain the peak load and calculate the fracture parameters such as fracture energy. Owing to the heterogeneity of concrete, the load-CMOD curves measured by the same set of specimens are different. For each investigated temperature and age, only the averaged curve of the same group of specimens was presented to avoid confusion. The overall load-CMOD curve is smooth, demonstrating that the unloading process was slow after the peak load and the test was carried out in a stable test state. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Curing Temperature** & **5 °C** & **20 °C** & **40 °C** & **60 °C** \\ \hline
**Compressive strength (MPa)** & 31.83 & 34.64 & 36.79 & 36.44 \\
**Elastic modulus (GPa)** & 23.36 & 26.72 & 29.26 & 28.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of concrete under different temperature at 28 d.  
Figure 2: The fracture experiment set-up. As presented in Figure 3, at the beginning of loading, there was a linear relationship between load and CMOD, indicating that concrete was in the stage of linear elasticity. The specimen did not undergo significant nonlinear deformation before the crack initiation or reaching the cracking load. For each curing condition, the initial slope increased as the concrete become older, which was attributed to the increase of the concrete strength and stiffness with time elapsing. Further, with the increase of curing time, the peak load grew markedly under each investigated curing temperature. For instance, for the specimens exposed to 5 \({}^{\circ}\)C, the peak load was 2.683 kN after 3 d, 2.906 kN after 7 d, 3.156 kN after 14 d, 3.427 kN after 28 d, and 3.911 kN after 60 d. The peak load increased by 14.1% from 28 d to 60 d, indicating that there was still a lot of un-hydrated cement inside the concrete under this curing condition. A similar phenomenon was also observed when the concrete kept at high temperature, but the rate of growth was lower than that of the concrete exposed to low temperature. Typically, for the concrete exposed to 60 \({}^{\circ}\)C, the peak load improved from 3.565 kN after 3 d of hydration to 4.491 kN after 60 d of hydration, where the peak load hardly increased beyond 28 d hydration. Figure 3 also reveals that the peak load improved with the growing curing temperature during an early age, but this situation did not hold in later ages, with the greatest ultimate values of peak load corresponding to the specimens kept at 40 \({}^{\circ}\)C. In detail, at 3 d, the peak load improved from 2.683 kN at 5 \({}^{\circ}\)C to 2.723 kN at 20 \({}^{\circ}\)C, 3.366 kN at 40 \({}^{\circ}\)C, and finally to 3.565 kN at 60 \({}^{\circ}\)C with a final ascent of 32.9%. However, at 60 d, the peak load of concrete stored at 5 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C and 60 \({}^{\circ}\)C was 3.911 kN, 4.092 kN, 4.781 kN, and 4.491 kN, respectively. The peak load of concrete exposed to 60 \({}^{\circ}\)C was only 12.9% and 8.9% greater than that of concrete kept at 5 \({}^{\circ}\)C and 20 \({}^{\circ}\)C, respectively, and even was 6.5% smaller than that of concrete exposed to 40 \({}^{\circ}\)C.  
Figure 3: Load-crack mouth opening displacement (CMOD) curves under dry environments with different temperatures: (**a**) at 5 \({}^{\circ}\)C; (**b**) at 20 \({}^{\circ}\)C; (**c**) at 40 \({}^{\circ}\)C; (**d**) at 60 \({}^{\circ}\)C.  
Besides, the post-peak curvature of the curve, which represented the brittleness of the concrete, was related to the curing condition and age. This can be attributed to the improvement of the interfacial transition zone (ITZ), in which the stress concentration was decreased and more new hydration products were accumulated as the curing age was prolonged.