## 3 Results
### Elasticity Modulus  
For each specimen, a stress-strain relation can be established for each of the four strain gauges. Consequently, the elasticity modulus and the stress-strain ratio can be calculated. The elasticity modulus is calculated in a linear region when the stress is between 10 and 45% of tensile strength. After the elasticity modulus has been calculated for each strain gauge, the average value is calculated. This value represents the elasticity modulus of the specimen, and the determined values are shown in Table 2 and presented in Figure 5.  
The results obtained in this work disagree with those in [38], which show that the elasticity modulus of unreinforced concrete shows significant development in the first 5-10 days and thereafter remains near-constant over time. Unlike the work in [38], the elasticity modulus is observed here to increase by 6.5% between the 9 th and 21 st day. Hence, it can be said that the carbon fibers contribute to increasing the elasticity modulus up to the 21 st day of age.  
Figure 5: Temporal development of elasticity modulus.  
However, the carbon fibers also influence elasticity modulus development after 21 (or 28) days of age. The results presented in [39] show that for unreinforced concrete, not having been cured for the whole time of its aging, the elasticity modulus will significantly decrease after the 21 st (or 28 th) day. The results in this work show elasticity modulus either insignificantly decreases after the 21 st day (7%, 0.6%, and 2% for 28, 35, and 49 days of age) or slightly increases (2.85% for 90 days of age). The elasticity modulus at the age of 70 days is equal to that at the age of 21 days. Thus, it can be said that carbon fibers prevent degradation of the elasticity modulus caused by concrete drying (shrinkage) [39] after 21 days of age and allow it to remain almost constant over time.  
The results for prediction of elasticity modulus development over time using fib Model Code 2010 [36] are shown in Figure 5 and indicate that this code cannot be successfully used to predict the development of CSFRC elasticity modulus over time. Thus, in this work, two equations for such a prediction are proposed. One equation is for prediction of the elasticity modulus of CSFRC at the age of 28 days, and the other at the age of 21 days. Both equations are a modification of fib Model Code 2010 equation [36].  
In doing so, Equation (8) aims to predict the elasticity modulus development of CSFRC over time based on the elasticity modulus at the age of 28 days:  
\[E_{c}(t)=\left\{e^{0.0841\cdot[1-(2\
icefrac{{1}}{{t}})^{1.55}]}\right\}\cdot E _{c}(28), \tag{8}\]  
whereas Equation (9) intends to predict the elasticity modulus development of CSFRC over time based on the elasticity modulus at the age of 21 days:  
\[E_{c}(t)=\left\{e^{0.0123\cdot[1-(2\
icefrac{{1}}{{t}})^{2.15}]}\right\}\cdot E _{c}(21), \tag{9}\]  
where  
\(E_{c}(t)\) is elasticity modulus of CSFRC at the age of (t).  
The prediction results calculated using Equations (8) and (9) are shown in Figure 5. It is clear that there is good agreement between the experimental results and those obtained through (8) and (9). The disagreement in results for Equation (9) at the ages of 28, 35, 49, 70, and 91 days is 7.6%, 1.4%, 3.1%, 1.1%, and 1.6%, respectively, and can be ignored. Equation (8) underestimates elasticity modulus at the ages of 14 and 21 days (0.9% and 1.1%) while the differences are lower after other numbers of days, except 91, than for Equation (9). At the age of 91 days, Equation (8) underestimates the elasticity modulus up to 2.2%. According to the obtained results, both equations can be successfully used for the prediction of CSFRC elasticity modulus development.  
Temporal CSFRC elasticity modulus development can be expressed regarding its tensile strength at the age of 21 days according to Equation (10):  
\[E_{c}(t)=\left\{e^{0.0123\cdot[1-(2\
icefrac{{1}}{{t}})^{2.15}]}\right\}\cdot [f_{c}(21)]^{1.344}. \tag{10}\]