# Effect of Fibre Reinforcement on Creep in Early Age Concrete
## 3 Results and Discussion
### Modulus of Elasticity  
The specimens loaded 24 h after their manufacture were subjected to a tension equivalent to 78% of their compressive strength. Figure 10a shows the stress/strain curve of the FRC specimens loaded at 24 h and at 3 d. As expected, for the same compression stress of 15 MPa, the deformation of the loaded specimens at 24 h is much greater than that corresponding to the specimens loaded at 3 d. In the case of the FRC specimens loaded at the age of seven and twenty-eight days, the stress level during the creep test was similar, equivalent to 53 and 47% of their compressive strength respectively. In Figure 10b, we can see the similarity of the stress/strain curves of the specimens loaded at the age of seven and twenty-eight days.  
Figure 9: Creep deformation undergone by FRC specimens: FRC loaded at 24 h (**a**); (**b**) 3 d; (**c**) 7 d; (**d**) 28 d.  
From the analysis of the stress/strain curves of the specimens during their loading, the modulus of elasticity of the FRC is obtained at the time corresponding to the start of the creep test. As expected, and according to the values obtained for the compressive strength of concrete, the value of the modulus of elasticity increases with the age of the concrete, as reflected in Figure 11.  
It can be observed that the modulus of elasticity evolves with age in a similar way to the evolution of the compressive strength (Figure 5).  
Figure 12 shows the stress/strain curves of the specimens during their unloading.  
Figure 10: Tension-deformation curve during loading of the specimens: (**a**) FRC specimens loaded at 15 MPa; (**b**) FRC specimens loaded at 20 MPa.  
Figure 11: Evolution of the modulus of elasticity of FRC with age.  
Figure 12: Stress/strain curve during the unloading of the specimens: (**a**) FRC specimens loaded at 15 MPa; (**b**) FRC specimens loaded at 20 MPa.  
Once the creep test was completed, the analysis of the stress/strain curves of the specimens during their unloading (Figure 12) makes it possible to obtain the residual modulus of elasticity of the FRC after the 384-day duration of the creep test. In the case of the residual modulus of elasticity of FRC, it is observed that its value decreases with the increase in the loading age of the concrete.  
As can be seen in Figure 13, the FRC specimens with the highest stiffness are those that were loaded 24 h after their manufacture.  
Performing a comparative analysis of the stress/strain curves of the specimens during their loading with the curves corresponding to their unloading, it can be observed that the influence of the creep of FRC on its deformation behaviour is different for concrete loaded at early age than for concrete loaded with an age greater than 3 d. Whereas in a concrete loaded at an early age, its modulus of elasticity increases after having been subjected to a creep episode, in concrete loaded at an age greater than 3 d, its modulus of deformation remains unchanged (Table 5).  
The difference between the initial and residual modulus of elasticity depends on the loading age. Furthermore, the two moduli of elasticity evolve in an inversely proportional way, so early loaded specimens have lower initial modulus of elasticity and higher residual modulus of elasticity.  
Figure 13: Evolution of the modulus of elasticity of FRC with its loading age.  
\begin{table}
\begin{tabular}{c c c} \hline
**Loading Age (d)** & **Eo (GPa)** & **Er (GPa)** \\ \hline
1 & 21.913 & 41.712 \\
3 & 33.500 & 38.842 \\
7 & 33.667 & 33.904 \\
28 & 36.100 & 34.918 \\ \hline \end{tabular}
\end{table}
Table 5: Evolution of the modulus of elasticity of FRC after a creep episode. Initial “Eo” and residual “Er” modulus of elasticity.