# Fracture Properties of Concrete in Dry Environments with Different Curing Temperatures
## 4 Discussion  
As mentioned earlier, during the first 3 d, the \(G_{\text{F}}\) and \(K_{\text{IC}}\) of concrete improved quickly with increasing age owing to the fast hydration rate of cement in this stage. The initial growth rate of concrete fracture parameters increased progressively and then reduced after maximizing, accompanying a steeper curve. As the curing temperature increased, the duration for this stage became shorter. In the later stage, the rate of hydration went down, resulting in that the concrete fracture parameters developed overtime at a smaller growth rate. In general, there should be the third stage, in which the fracture parameters of concrete tended to their stable values, as the age was further extended. However, at low temperatures, the third stage was not seen or was not notable because this study only investigated the age of up to 60 d, meaning that there was a lot of un-hydrated cement inside the concrete under these curing temperatures. This variation of concrete fracture parameters regarding the age was consistent with other independent investigations [34]. For example, [PERSON] et al. [35] found that with the increase of test age from 3 to 90 d, \(G_{\text{F}}\) and \(K_{\text{IC}}\) increase from 0.961 MPam\({}^{0.5}\) to 1.448 MPam\({}^{0.5}\) and from 99.7 N\(\cdot\)m\({}^{-1}\) to 126.5 N\(\cdot\)m\({}^{-1}\), respectively. [PERSON] et al. [36] demonstrated that the fracture energy shows a rapid increase during earlier and then starts to converge to a certain extent of 173.1 N\(\cdot\)m\({}^{-1}\) at 28 d. The main reason for the increase of concrete \(G_{\text{F}}\) and \(K_{\text{IC}}\) with age was the strengthening of the ITZ between the paste and aggregate. ITZ is the most sensitive area inside the concrete, where the largest number of micro-cracks are formed [37]. In other words, the failure of concrete usually depends on the strength of its weakest area. At an early age, there was a lot of un-hydrated cement in the ITZ, and it resulted in high porosity in this zone [38]. The bridging effect of the aggregate was very strong causing the cracks to propagate along with the interface, and a large number of aggregates were extracted from the matrix, as presented in Figure 6a. As the reaction of cement hydration progressed, an increasing number of hydration products accumulated in the ITZ and the ITZ pores were stuffed with hydration products, indicating that the size and content of pores in ITZ and cement paste decreased. The ITZ and cement paste also became stronger. As a result, the bridging effect of the aggregate became weaker and a growing number of aggregates were broken. The fracture mode under load also changed from around aggregate or bond zones through the aggregate directly, as presented in Figure 6b. Further, the aggregate strength was greater than that of the ITZ and cement paste, demonstrating that more energy was needed in order to get over the enhanced ITZ and cement paste and the aggregates. Consequently, the greater values of fracture energy and effective fracture toughness were obtained at later ages. However, as the strength of the ITZ and cement paste further improved, the fracture pattern did not alter dramatically, where an only increasing number of aggregates were extracted. Therefore, effective fracture toughness and fracture energy did not increase significantly, but gradually tended to their stable values. More importantly, as is evident from the data in Figures 5 and 6, the effective fracture toughness and fracture energy of concrete cured at elevated temperatures developed at a faster speed at the early stage. For instance, after 3 d of hydration, the \(G_{\text{F}}\) and \(K_{\text{IC}}\) of concrete cured at 60 \({}^{\circ}\)C was 20.9% and 21.1% larger respectively than that of concrete cued at 20 \({}^{\circ}\)C. This promotion effect of temperature on the early fracture parameters of concrete is related to the accelerated hydration reaction rate of cement, and the finer pore distribution and denser microstructure [39].