# Fracture Properties of Concrete in Dry Environments with Different Curing Temperatures
## 4 Discussion
Over time, however, the rate of growth of the fracture parameters of concrete kept at high temperatures also decreased at a higher rate, which resulted in these samples obtained the smaller ultimate values of \(G_{\text{F}}\) and \(K_{\text{IC}}\) than those of samples kept at reduced temperatures. For example, after 60 d of hydration, the difference in the \(G_{\text{F}}\) and \(K_{\text{IC}}\) between the samples exposed to 60 \({}^{\circ}\)C and 20 \({}^{\circ}\)C was reduced to 4.2% and 3.3%, respectively. Even the values of \(G_{\text{F}}\) and \(K_{\text{IC}}\) at 60 \({}^{\circ}\)C were less than that at 40 \({}^{\circ}\)C beginning from the 43 rd day and 41 st day, respectively. Further, the growth rate of fracture parameters at 5 \({}^{\circ}\)C and 20 \({}^{\circ}\)C also indicated that with the further extension of curing time, the curves at 5 \({}^{\circ}\)C and 20 \({}^{\circ}\)C will doubtlessly cross the curves at the higher temperature. Namely, although a higher temperature accelerated the rate of hydration reaction of cement and promoted the development of \(G_{\text{F}}\) and \(K_{\text{IC}}\) in the early stage, it harmed the development of the ultimate value of fracture parameters of concrete. The \(G_{\text{F}}\) and \(K_{\text{IC}}\) of concrete showed a temperature crossover effect, and the optimum temperature propitious to their development was approximately 40 \({}^{\circ}\)C. Such a trend was in line with previous observations found by other scholars with a slightly different method. [PERSON] et al. [40] claimed that curing at high temperatures was conducive to rapid strength gain, but at later ages, the greatest strength was measured by concrete cured at 20 \({}^{\circ}\)C. [PERSON] [41] found that the concrete compressive strength cured at 50 \({}^{\circ}\)C was 7% smaller than that of concrete stored at normal temperature after 28 d. [PERSON] et al. [42] demonstrated a 15% reduction in the concrete compressive strength cured at 60 \({}^{\circ}\)C after 14 d compared to that cured at 40 \({}^{\circ}\)C. [PERSON] et al. [43] clarified that after 28 d the tensile strength improved with the pouring temperature up to 32 \({}^{\circ}\)C, but decreased these properties as a further increase in temperature. The adverse effect of high-temperature on the later-age fracture properties of concrete makes us wonder: What happened at the microscopic level at a high-temperature? We concluded that this adverse effect of high-temperature on the effective fracture toughness and fracture energy of concrete was related to some different mechanisms. Firstly, this phenomenon is caused by the non-homogeneous dispersion of hydration products inside the pores of the hardened cement paste [44]. The hydration reaction of cement was accelerated at elevated temperature, so there was neither sufficient time for the hydration products to disperse surrounding nor sufficient time for them to precipitate and distribute evenly. Consequently, more and more hydrated products were concentrated surrounding the hydrating or un-hydrated particles of cement, resulting in a more non-homogeneous distribution of hydrated products and causing the concrete to become a porous structure [45]. Secondly, the change in the C-S-H apparent density at high-temperature was another important reason for the decrease of later-age values of concrete fracture parameters [46]. [PERSON] et al. [47] clarified that there was a remarkable  
Figure 6: The fracture surface of concrete at different ages: (**a**) at 3 d; (**b**) at 60 d.  
enhancement in the C-S-H apparent density in the situation of elevated temperature, which came from the different assembly of the C-S-H. Owing to this increase, the filling space of C-S-H was reduced, leading to that the microstructure of the concrete was much coarser and porous, which had a detrimental effect on the concrete fracture parameters.